In the transmission of digitally encoded voice, it is important to maintain synchronization between the two end points so that no digital information is lost due to differing rates of transmission and reception. Synchronization is the ability to maintain a stable frequency and precise timing to allow digital transmission services to read data out and read data into the transmission system at the same rate. Without synchronization, rates differ and data slippage occurs resulting in data being lost. Within the prior art, circuit switch networks and packet data switching networks when operating independently of each other have solved this problem in the following manner. In circuit switched networks, synchronization is centrally located and is synchronized throughout continental United States. For example, long distance transmission carriers, such as AT&T, have placed synchronization technologies in there central offices and relied on T1 trunk-based recovery network timing subsystems to synchronize data being received from the network. Packet switched network have allowed the receiving endpoint to signal the transmitting endpoint to slow or speed-up the transmission rate. This type of control is utilized in asynchronous transfer mode (ATM) and frame relay transmission (FR). However, the internet protocol (IP) transmission systems provide no such synchronization mechanism even though they are packet switched networks.
The prior art methods for achieving synchronization in circuit switched networks and packet switched network performed well if the two types of networks were not interconnected. An exception to this situation was in the situation where ATM or frame relay was utilized with a circuit switched network with the same data transmission company controlling both systems. Within the present business communication switching environment, there exists a need for simplified maintenance, management, and access to voice information on diverse networks. This need is forcing the convergence of a variety of circuit switched and packet switched networks. In addition, a new class of real-time multimedia networks is emerging that will also require synchronization.
The combination of a circuit switched network and a packet switched network is referred to as a hybrid network. Hybrid networks that lack synchronization exhibit the same symptoms as if packets were being lost within a packet switching system with some asymmetry. (1) If the read-out is faster than the read-in, eventually the reader exhausts the jitter-buffers and must wait for them to refill. The voice coder sees an empty stream of voice information and hence the voice quality suffers remarkably. (2) If the read-out is slower than the read-in, eventually the jitter-buffers fill full, and new packets are discarded. The voice coder sees a loss of packets and again the voice quality suffers. If the buffers are made too large, the delay in transmitting voice information from one person to another person is increased. It is well known that a large delay in voice transmission is objectionable to people. The delay is increased as the buffers are made larger, because the speech samples spend more time in the buffers.
A prior art solution for interconnecting a hybrid network is illustrated in FIG. 1. Synchronous physical (PHY) interface 101 is reading out PCM voice samples to voice coder 106 via path 114. Voice coder 106 transmits these PCM packets via path 113 to IP switched network 107. IP switch network 107 transmits packets containing PCM samples to voice coder 106 which transmits these to PHY 101 via elements 102, 103, and 104 and paths 108, 109, and 111. PHY 101 utilizes insert/remove circuit 102 to obtain the packets that are being placed in sampled queue 104 by voice coder 106. Insert/remove circuit 102 adds or deletes PCM samples as required to maintain a synchronous transfer of data to PHY 101. Insert/remove circuit 102 performs this activity by utilizing low energy detector 103. Low energy detector 103 evaluates the PCM sample that will next be transmitted from sample queue 104 to circuit 102 via path 109. Low energy detector 103 indicates to circuit 102 if the energy contained within the PCM sample is below a predefined threshold and may be discarded. If there is not a sample present in sample queue 104 and a sample is required to be transmitted to PHY 101, insert/remove circuit 102 transmits a low energy PCM sample. When insert/remove circuit 102 has to delete samples being received from sample queue 104, circuit 102 deletes any present sample indicated by low energy detector 103 as being below predefined energy value requirement. Circuit 102 commences this operation at some predefined capacity of sample queue 104. The problem with this prior art solution is that insert/remove circuit 102 has no knowledge of the number or location of PCM samples that are below the predefined energy value within sample queue 104. Hence, for example, if circuit 102 determines that it must delete five PCM samples, circuit 102 will delete the next five PCM samples that low energy detector 103 indicates are below the minimum energy level. This can result in deletion of samples over a small period of time and cause deterioration of the voice quality being produced by PHY 101.